Just like that, at precisely 11 p.m. on Thursday, minutes after the end of the ninth day of the Senate trial of Donald John Trump, Senator Lamar Alexander ended it. In a statement tweeted out by his office, the Tennessee Republican said that the President was guilty of “inappropriate” pressure on Ukraine in the service of his own reëlection. The House Democrats managing the case had “proven” it, but that was not enough to impeach and remove Trump from office. Nor was it enough to continue the trial, Alexander said. He would not support calling witnesses. He would not support any effort to obtain further evidence. He did not want to hear even from John Bolton, the former Trump national-security adviser who is prepared to testify that the President directly admitted to the central allegation in the impeachment case. Without Alexander’s support, the trial almost certainly cannot continue. Democrats do not have the fifty-one votes they need to call witnesses, and so, sometime in the coming days, the third Presidential-impeachment trial in American history is virtually certain to reach its preordained conclusion: a partisan acquittal by the Republican-controlled Senate, following a partisan impeachment by the Democrat-controlled House.

In the end, it’s no small irony that Trump was saved from embarrassing public testimony against him by one of the last representatives of the Republican establishment that so recently scorned him—and for which the President himself has nothing but scorn. Alexander declined to endorse Trump in 2016, and had previously bucked the President on trade, health care, and his much vaunted border wall. But as Alexander retires, later this year, after decades of service once characterized by bipartisanship, his most decisive final act will have been to do Trump an enormous favor. Alexander’s mentor in politics, Senator Howard Baker, is remembered as the Republican leader who pursued the facts about Richard Nixon during Watergate and demanded answers to the key question of what Nixon knew and when he knew it. Lamar Alexander will not have such an honor. He will go down in history as the Republican senator whose choice at a pivotal moment confirmed the complete and final capitulation of the G.O.P. to the crass New York interloper in the White House.

Alexander’s late-night statement was no real surprise. The “closest friend” to Senate Majority Leader Mitch McConnell—as McConnell made sure to point out to the Times, earlier this week—Alexander ended up where most Senate Republicans were always expected to end up. He criticized Trump but refused to vote to remove him from office. After making that decision, Alexander went a step further and said that there was no real need to hear any of the evidence that Trump has so far successfully ordered his Administration not to provide. Even the last-minute revelation, on Sunday night, in the Times, of Bolton’s unpublished manuscript, could not sway Alexander; he knew enough.

Right up until that oh-so-predictable end, though, Alexander played it coy. Perhaps he was loving his final moment in the spotlight. Perhaps he really was undecided. As Thursday’s proceedings began, virtually all of the other senators had declared their positions, and only four Republicans appeared to be genuinely considering voting for witnesses. Senators Mitt Romney and Susan Collins were likely to vote yes, which left Lisa Murkowski and Lamar Alexander. Even if Murkowski, who pronounced herself Bolton-“curious” earlier in the week, would vote yes, Democrats needed both Murkowski and Alexander to get to fifty-one; three Republican yeses would only yield a 50–50 tie. And, as night fell on the capital, pretty much no one believed that Supreme Court Chief Justice John Roberts, presiding over the trial, would be willing to cast the deciding vote.

By the dinner break on Thursday’s long session of questions from the senators to the duelling legal teams, it was clear that it would all come down to Alexander. Until this point, Alexander had been a complete “cipher,” as Chris Coons, the Delaware senator who talks with his Republican colleagues more than most Democrats, told me. But just before dinner Alexander began to tip his hand, as he submitted his first question of the trial alongside two staunch defenders of Trump: Ted Cruz, of Texas, and Steve Daines, of Montana. The group asked the House managers to address the partisanship of Trump’s impeachment and contrast it with that of the Nixon and Clinton impeachment processes. Alexander and Cruz together? You didn’t have to be a Senate veteran to have a sense of where things were headed.

During the recess that followed, Alexander spoke to reporters and promised that he would announce his vote in a few hours, when the question time was over. Then he met privately with Murkowski. Speculation grew feverish. Not long after the Senate reconvened, a reporter tweeted that Alexander had sent a Senate page to McConnell’s desk with a note, which McConnell then read and put in the pocket of his suit jacket—without ever looking at Alexander. Soon after that, Murkowski sent a question to Chief Justice Roberts to read. It seemed to reveal where she was leaning, at least: “This dispute about material facts weighs in favor of calling additional witnesses with direct knowledge. Why should this body not call Ambassador Bolton?”

The spotlight returned to Alexander, who probably had not received this much national media attention since 1996, when his brief Presidential candidacy gained notice largely because of his distinctive flannel shirts and a campaign logo that consisted only of his first name and an exclamation point. Reporters up in the Senate press gallery trained their eyes on Alexander and reported that he chatted amiably at various points with the Ohio Republican Rob Portman, the Washington State Democrat Maria Cantwell, and the President’s lawyers. Alexander was observed reading a book, “Impeachment: An American History,” an account of the three previous Presidential impeachment proceedings in U.S. history. (My husband, Peter Baker, wrote the chapter on the Clinton impeachment and trial; I agree with Alexander’s endorsement that it is “a very good book.”) After reading for a while, Alexander then turned to his seatmate, Mike Lee, of Utah, and shared a passage with him. What did it all mean?

Around 10 p.m., Alexander and Murkowski joined with another fervent Trump critic turned defender in the Senate, Lindsey Graham, to pose a question to Trump’s defense team. “Isn’t it true,” they asked, that, even if Bolton testified and everything he said was accurate, it “still would not rise to the level of an impeachable offense, and that therefore his testimony would add nothing to the case?” Sensing where this was going, Trump’s lawyer Patrick Philbin hastened to agree.

“It’s over,” one Democratic senator said to another, according to a reporter in the gallery. And, indeed, it was. The question offered a preview of the Alexander statement to follow. A few minutes later, Jerry Nadler, the chairman of the House Judiciary Committee, gave a truncated closing statement that suggested that he, too, knew what was about to happen. “They are afraid of the witnesses,” Nadler said. “They know Mr. Bolton and others will only strengthen the case.” On that note, the trial adjourned at 10:41 p.m. Nineteen minutes later, Alexander’s office tweeted out his statement. Murkowski did not join in, at least not yet. “I am going to reflect on what I’ve heard, reread my notes, and decide whether I need to hear more,” she told reporters; her office said she would announce her decision on Friday morning. Her colleague Susan Collins, meanwhile, announced that she would vote yes for the witnesses. Mitt Romney followed suit first thing Friday morning. But how much did it matter?

All fifteen previous impeachment trials in the U.S. Senate, including the two previous Presidential-impeachment trials, had witnesses. But Lamar Alexander has spoken. Donald Trump’s stonewalling will succeed where Nixon’s failed. Perhaps Alexander has done us all a favor: the trial that wasn’t really a trial will be over, and we will no longer have to listen to it. The Senate can stop pretending.

Four scholars of constitutional law—Noah Feldman, Pamela Karlan, Michael Gerhardt, and Jonathan Turley— answered questions from the House Judiciary Committee on what constitutes an impeachable offense. Here are the highlights from Wednesday.